When cables are downsized without a fuse, question

In most circumstances, fuses or other OCPD must be sized to protect the cable, there are however a number of exceptions.

o.75mm flex is allowed for pendant drops on lighting circuits. This has a good record in practice, how many light fittings, even under fault or failure conditions, can draw more than the current rating of the flex.

Research has shown that 0.75 mm flex can be protected against short circuit by a fuse of up to 16 amps or an MCB of up to 20 amps. Overload protection is due to the design of the light fitting. In the exceptional case of a lighting circuit fused at more than 20 amps, then in my view 0.75mm flex should not be used.

Similar arguments apply to flexible cords on portable appliances, these are commonly 0.75mm on small appliances, and are protected against short circuit by a 13 amp plug fuse or if non fused plugs are used are protected by a circuit fuse of 16 amps or by an MCB of 20 amps, normal practice in most of the world.

As regards a 2.5mm flex or able supplying an oven on a 32 amp circuit, this should be fine since the design of the oven protects against overload, and the 32 amp MCB protects against short circuit.

In fixed installations, apparently undersized cables are allowed when the design makes overload very unlikely. For example a 2.5mm cable can supply one twin 13 amp socket from a 32 amp circuit. Overload is very unlikely since each plug is fitted with a fuse not exceeding 13 amps, and in practice the long term load is unlikely to exceed 20 amps.

Jimi said:

Also, yes a low impedance short circuit or earth fault will trip the MCB/RCBO/RCD quickly if the Zs is low enough, but what if there was a fault letting through enough current to melt the small cable but not to trip the MCB/RCBO/RCD?

So, BS 7671 has two requirements for overcurrent protection:

The first is protection against fault current. In this situation, a fault of negligible impedance only is considered. The cross-sectional area (csa) of the cable must meet the adiabatic equation, although a more complex alternative calculation is also permitted.

The second is protection against overload current. Overload current may be omitted under certain circumstances, usually for final circuits where accidental overload is not considered possible (usually fixed resistive loads).

BS 7671 does not consider resistive faults in the way you describe - however, some equipment, such as those with certain heating elements, may occasionally develop faults which might lead to overload rather than full fault current. In some cases, the manufacturer of an appliance may specify the maximum rating of the overcurrent protective device for the appliance, and if that is the case, it might be appropriate to rate the final circuit cable according to the overcurrent protective specified by the manufacturer, if you are in any doubt.

Bear in mind also that a flex usually has a larger cpc CSA than the equivalent T&E. For example a 6mm² T&E has a 2.5mm² cpc, as does a 2.5mm² flex. so if the OCPD is suitable for a fault current on the T&E's cpc, it should be suitable for the flex too.

433.3 and 560.7.3 come to mind.

Z.

The point to consider is the PSCC at the size transition. As I mentioned the other day, this is likely to be considerably smaller than that at the CU, and it makes a big difference to the adiabatic equation result. However, as Broadgage has said this size change is not a known point of failure in real installations, and this is because the equation works to help you in this case. As I have said many times before, cable current ratings are not the point where correct operation changes to complete failure, and there are many "if, but and then" points, all of which are considered to be worst-case at once. The current rating as tabulated is for cables loaded 24/7 at the maximum rating and at a particular ambient temperature. Diversity works because the cables take a very significant time to reach maximum temperature, and appliances like ovens and hobs do not take power continuously. In reality, the cable ratings for any domestic installation except electric heating are VERY conservative for all the above reasons. Just think a moment, when was the last time you touched a cable that was more than slightly warm, at 70C conductor temperature you would have let go fairly quickly, the answer is probably never in a domestic. A useful addition to the design guide would be a section to fully explain cable rating, protective device rating, and diversity, all of which are very badly understood by most electricians. You will see this illustrated by the endless discussion of ring final circuits and the alleged danger these present. Has anyone ever seen a melted cable on an RFC correctly installed? If so I would love a picture and full details of the installation. This could change if those who install cables in cavities continue their work, and insulation levels continue to increase. I consider this very bad practice, although it is cheap and easy, and saves much wall chasing.

I have come across many, many electric shower cables buried under lots of loft insulation, but have never seen any damaged insulation due to over heating in the main run. I have though seen many damaged cable ends at 45 Amp cord switches where the terminals were not tightened fully or became lose when the installer struggled to get the switch in position next to its box.( I squared R heating effect.)

Z.

Accessory terminals can be something else - quite often the current that we can think of as coming down the wire as  a more or less full cylinder of current, i.e using the full area of conductor, is asked to bunch up to one side or the other to get on or off via the small area at the point of a screw and the rather odd shaped splodge just opposite it where two curved surfaces meet, somteimes  with very different radii.
There is a reason that very high current terminals use flat palm braids at high frequencies or an all-round hex crimp and a flat bolt area for DC/50Hz stoff , to be sure that the contact area is several times the cross-section of the cable coming in, so you see no pinch points if you  cut across the current anywhere along its journey.

Mike

From IET guide 1:

Some loads, due to the nature of the load itself, cannot present an overload and these loads can be provided with fault current protection only. As an example, for the flexible cable of a luminaire pendant, protection against fault current only is usually sufficient.

Can you give any examples of when an overload could actually happen on equipment?

I'd have thought that all plug-in equipment will be designed with the cable CCC > max load drawn by the equipment, and they already come with a fuse anyway. I'm thinking overload can't happen at the end of a circuit where equipment is connected if equipment's cable CCC > max load drawn by the equipment. Only on the main circuit supplying lots of loads (e.g. socket ring final), due to diversity being applied, giving the potential for overload. The only thing I can think of is people modifying equipment? Or damage to equipment causing higher current draw maybe?

The obvious place where we like to provide overload protection would be equipment with motors especially when doing things that may be stalled, like wood chippers or rubbish crushers because the exact mechanical load is rather indeterminate.

Mike

(and so on all but the smallest motors this is indeed required.)

On plugged in appliance it's different as there's the fuse to create a new Ib<In<It (or max load<fuse<It).

Also overload protection (i.e. heavier than anticipated currents flowing in an otherwise healthy circuit) may be provided downstream of the conductor it's protecting as the same current will be flowing in the protective device as in the conductor. That's often the case with industrial motor circuits where the fuse/MCB in the distribution board is rated far higher than the cable and overload protection is provided next to the motor by a special motor overload device (that was it can be more precisely matched to the motor's characteristics and have the convenience of being able to reset it locally). You also see the same principle on domestic ring final circuits where 20A cable (usually 2.5mm²) may be used on a spur from a 32A circuit as long as it's to a solitary socket - the assumption being that the total load on even a double socket won't exceed 20A for any significant length of time.

The fused 13A plug is itself rather an oddity - in most of the rest of the world domestic plugs are entirely unfused - so 0.75mm² even on a 16A or 20A protective device is very common. Generally it's up to the appliance to prevent overload so the upstream protective device only has to protect the flex from short-circuits - which generally it will do very successfully - at least as far as preventing gross overheating or fire - even if occasionally the flex isn't serviceable afterwards - but given that the appliance it belongs to is already faulty, that isn't a huge loss.

We only really need fused plugs because we like 30A or 32A protection for socket circuits - and those larger devices are a lot worse at protecting small flexes from faults.

   - Andy.